Fixed valgrind error in DBTest.CompressedCache

Summary:
Fixed valgrind error in DBTest.CompressedCache.
This fixes the valgrind error (thanks to Haobo). I am still trying to reproduce the test-failure case deterministically.

Test Plan: db_test

Reviewers: haobo

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13899

db/db_test.cc

@@ -1797,12 +1797,12 @@ TEST(DBTest, CompressedCache) {
     // Write 8MB (80 values, each 100K)
     ASSERT_EQ(NumTableFilesAtLevel(0), 0);
     std::vector<std::string> values;
-    Slice str;
+    std::string str;
     for (int i = 0; i < num_iter; i++) {
       if (i % 4 == 0) {        // high compression ratio
         str = RandomString(&rnd, 100000);
       }
-      values.push_back(str.ToString(true));
+      values.push_back(str);
       ASSERT_OK(Put(Key(i), values[i]));
     }